In the current typical car body finishing, an electrodeposition coating is first applied to the substrate steel surface and baked and, then, an anti-chipping primer, an intermediate coat, a base coat, and a clear top coat are applied in succession.
In recent years, much research has been undertaken for establishing a coating system using water-based coatings in lieu of solvent-based coatings for reducing the release of VOC in the atmosphere and, from the standpoint of energy conservation, applying and baking the successive coats by the two-coat/one-bake method.
Japanese Kokai Publication Hei-6-9925 discloses an anti-chipping primer comprising an aqueous dispersion of an 2-alkenoic acid-ethylene copolymer and a water-based polyurethane resin as film-forming resin components. Japanese Kokai Publication Hei-7-252449 and Japanese Patent Application Hei-8-354881, for instance, discloses a water-based intermediate coating comprising a polyester such that a polyalkadienediol or hydrogenated alkadienediol has been incorporated in the polyester chain.
However, it has been discovered that when such an anti-chipping primer and an intermediate coating are applied by the two-coat/one-bake method to provide a chipping-resistant composite paint film, several troubles are inevitable.
The chipping-resistant composite paint film is used to protect the paint of the running car body against the injury caused by gravels sent flying and impinging on the body. Therefore, the anti-chipping primer is selectively applied to the strategic surface area in the lower zone of the car body which is usually subject to the impact of flying gravels. In actual coating practice, the anti-chipping primer is sprayed against said strategic area, with the result that there is created a discontinuous dot-pattern of the spattered anti-chipping primer around said strategic area where a continuous film is formed. When, thereafter, an intermediate coating is applied to the whole body surface inclusive of the continuous film area and discontinuous dot area in the wet-on-wet method and the two coats are baked together to provide a composite paint film, the overall appearance of the finish is sacrificed due to the presence of said discontinuous dot-pattern.
Under the circumstances, the inventors of the present invention modified an anti-chipping primer in such a manner that although it would form a discrete layer, independent of the subsequently applied intermediate coating layer, in the continuous film area, it would be at least partially compatible with the intermediate coating to form an integral layer in the discontinuous dot area, thus succeeding in realizing an improved appearance of the composite paint film.